The present invention relates to air compressors. More particularly, the invention relates to controlling the air provided to the suction side of a rotary screw type air compressor.
Conventional rotary air compressors have an inlet valve that controls air flow to the inlet or suction side of the compressor. The inlet valve throttles flow when load on the compressor is diminished and shuts fully when the load on the compressor is removed. The inlet valve is commonly referred to as an unloader valve. The compressor is loaded when the inlet valve is open permitting air to flow through the compressor inlet. The compressor is unloaded when the valve is closed to block flow through the compressor inlet.
Unloader valves are typically designed to prevent backflow through the compressor inlet. Backflow typically includes a mixture of air and oil and may occur when the compressor is stopped while the discharge side of the compressor is pressurized. Backflow is a problem because of the associated loss of oil to the system, and potential environmental contamination. One conventional method of preventing backflow is to provide a check valve in the air service and oil injection lines.
Conventional unloader valves may be opened and closed pneumatically. Pneumatically controlled unloader valves require a control air system for operation. Unloader valves may also be operated by a hydraulic system. Hydraulic unloader valves frequently leak hydraulic fluid and require replacement of parts, such as diaphragms, gaskets and the like. These conventional systems typically require operation of the valve against the air pressure associated with the compressor. For example, U.S. Pat. No. 5,540,558 to Harden discloses an unloader valve that is powered by an electric motor. To move the valve to the closed position, and place the compressor in an unloaded condition, the valve must be moved against the force of the incoming air being sucked into the compressor. The requirement to move the valve in opposition to the force of the incoming air complicates the control system for conventional unloader valves. Thus, there is a need for an unloader valve that may be operated easily without regard to the surrounding air flow.
The unloader valve reduces air flow entering the compressor to prevent excessive discharge pressure from occurring during a reduced load condition. As discussed above, when the compressor is completely unloaded, the unloader valve shuts. Destructive vibration of the compressor and excessive noise can occur when the unloader valve is completely shut and the compressor remains operational. The noise and vibration, commonly referred to as xe2x80x9crumblexe2x80x9d, occurs due to the mated screws continuing to rotate with no air flow.
Many compressors include xe2x80x9canti-rumblexe2x80x9d systems to prevent compressor damage. A typical anti-rumble system may include a recirculation path for discharge air back through the rotors while the unloader valve is shut. For example, U.S. Pat. No. 4,396,345 to Hutchinson discloses an anti-rumble system in which air is recirculated from a oil/air separator tank on the discharge of the compressor back to the compressor inlet. U.S. Pat. No. 4,406,589 to Tsuchida also discloses an anti-rumble system in which air from the discharge of the compressor recirculates through the compressor when the unloader valve is shut. These conventional anti-rumble systems typically require expensive pilot or solenoid valves that are typically mounted on the exterior of the compressor housing. Thus, there is a need for an anti-rumble system that is incorporated into the housing of the unloader valve.
In addition, most conventional compressors include a vent path to relieve excessive compressor discharge pressure. Like the anti-rumble system, the vent path removes air from the discharge side of the compressor. In conventional compressors, the vent path and anti-rumble system are separated, further complicating the design and increasing the amount of air piping and control systems. At present, the applicants are aware of no system combining the compressor discharge vent and anti-rumble system into a single system that uses the same compressor discharge pressure. Such an improvement would greatly simplify the current systems and reduce the manufacturing costs of the compressor.
The conventional unloader systems and compressors described above typically require several valves and associated operating systems to be mounted on the compressor casing. Thus, the moving elements of the systems are exposed to the environment. As compressors are operated in many harsh environments, such as, for example, sandy and dusty construction sites, these moving components can become dirty or damaged. There remains a need to develop a compressor unloader system with parts and components that are contained within the compressor casing.
According to one aspect of the present invention an inlet unloader valve for a rotary compressor is provided. The valve comprises a hollow sleeve having a circumferential bottom seating surface and a spring for biasing the sleeve downward toward a seated condition. The sleeve may be positioned so that air flowing into the compressor moves through the hollow interior of the sleeve and under the circumferential bottom seating surface. The valve may further include a seating member positioned below the sleeve so that the bottom seating surface contacts the seating member to close the valve. Sleeve type valves typically have low opening and closing forces when compared to more conventional poppet type valves. The seating member may be biased upwardly toward the sleeve. The valve may be configured to permit leakage past the valve when the valve is in a seated condition.
The valve may further comprise a centrally located guide rod disposed within the sleeve and extending along the longitudinal axis of the sleeve to thereby providing a guide when the sleeve moves in the axial direction. The sleeve may include a centrally located hub for receiving the guide rod. The valve housing may include a mechanical stop that contacts the sleeve and limits the upward motion of the sleeve. The guide rod may be secured to the valve housing. The valve may be air operated and may include an air passage for carrying control air to the bottom of the sleeve for forcing the sleeve in the upward direction to open the valve.
The present invention is also directed to a compressor anti-rumble system. The system may comprise a supply of pressurized air; a first passage positioned to vent the supply of pressurized air to the inlet of the compressor upstream of a compressor inlet valve; a second passage positioned to direct the supply of pressurized air to the inlet of the air compressor downstream of the compressor inlet valve; and a control valve positioned between the air supply and the first and second passages. The anti-rumble system may be configured so that the pressurized air includes oil that lubricates the compressor inlet valve. Preferably, the first and second passages are formed in a housing containing the compressor inlet valve. The compressor inlet valve may be a sleeve type valve.
In yet another alternative embodiment the invention is directed to an inlet air control system for an air compressor. The system may comprise an unloader valve positioned within an unloader housing to control the flow of air into the inlet of the air compressor; an anti-rumble control valve positioned in the housing between a supply of pressurized air and the inlet of the compressor downstream of the unloader valve. The system provides for a flow path for air from the supply of pressurized air through the anti-rumble control valve and into the air compressor to exist when the main unloader valve is shut and the compressor is operating.
The invention includes a system for controlling an air compressor unloader valve comprising a first control element for controlling the operational mode of the unloader valve, wherein the unloader valve operational modes include a shutdown mode and an automatic control mode, wherein the unloader valve is closed in the shutdown mode; and a second control element for controlling the position of the unloader valve when the unloader valve is in the automatic control mode. The first and second control elements may include solenoid valves.
The second control element may vary the position of the unloader valve based on the discharge pressure of the compressor and may include a control air system for supplying control air to operate the unloader valve. The second control element may be configured to close the unloader valve when compressor discharge pressure reaches a predetermined value. Preferably, the second control element includes an adjustment mechanism for selecting predetermined values of compressor discharge pressures. The adjustment mechanism may be electrical or mechanical in nature.
The present invention may also be directed to a system for operating a pneumatically controlled compressor unloader valve. The system may comprise a first solenoid valve located in a passage for carrying pressurized air to a control air system for operating the unloader valve. The control air system includes a control air passage for carrying the pressurized air to the unloader valve and a control valve for controlling the position of the unloader valve by controlling the pressure of the air in the control air passage. The control valve may be air operated and may be positioned to vent the control passage when the control valve is open.
Preferably, the pressurized air is supplied from the discharge of the air compressor so that the control valve opens when compressor discharge pressure reaches a predetermined value. The system may also include a second solenoid valve located in a passage for carrying pressurized air for biasing the control valve towards a valve seat.
In another alternative embodiment the present invention is directed to an air system for operating an unloader valve for a compressor. The air system includes a supply of pressurized air; a control air header for providing operating air to the unloader valve; and a control valve for controlling pressure in the control air header. The control valve is configured to open and connect the control air header to a vent path thereby causing the unloader to valve shut. The control valve is preferably spring biased in the closed position. The system may include a supplemental biasing mechanism to change the force that the spring applies to the control valve. The supplemental biasing mechanism may be a pressurized air system, mechanical linkage or an electrical linkage.
In yet another embodiment the invention is directed to a rotary screw type air compressor. The compressor may comprise an air operated unloader valve for controlling the flow of air from an air supply into the inlet of the compressor; a control air system for operating the unloader valve. The unloader valve may comprise a sleeve valve. The compressor may also include an anti-rumble system for supplying air to the compressor inlet when the unloader valves are shut and the compressor is running.
The anti-rumble system may further comprise a control valve located in a passage between the compressor discharge and a vent passage and between the compressor discharge and the inlet of the compressor so that when the control valve is open the compressor discharge is operatively connected to the vent passage and to the inlet of the compressor.
Yet another embodiment of the present invention is directed to a method of operating a rotary air compressor comprising the steps of closing a unloader valve at the inlet of the compressor; starting the compressor; providing leakage by the unloader valve during compressor startup; and opening the unloader valve when the pressure reaches a first predetermined value. The method may include the additional steps of shutting the unloader valve when compressor discharge pressure reaches a second predetermined value.
The present invention is also directed to a method of operating an air compressor unloader valve. The method includes the steps of applying a closing force to maintain the unloader valve in the closed position; pressurizing a control air header to open the valve, wherein the control air header supplies air to the unloader valve and applies sufficient opening force to the valve to overcome the closing force on the valve; and depressurizing the control air header to close the valve. The step of depressurizing the control header may occur at a predetermined value of air compressor discharge pressure.